Add developer MOTD (bug 945956)

Added MOTD to My Submissions page, along with page for editing it, permission for editing. Moved some CSS and JavaScript around to make existing MOTD code more general.

Squashed commit messages:
needs ACL
implemented motd for My Submissions page
added menu to set developer motd
added migration
since we're testing (1) initial value, (2) error, and (3) success, those could all be done as form tests. I agree in this case. the form is small enough. besides, there's a [`set_config` call](#diff-b1198da566db58098d64ca41cc26001fR307) in the view that has to happen for the MOTD to be saved successfully. if we [moved this to the form](#diff-b1198da566db58098d64ca41cc26001fR307), we could test the form alone.

if we can at least break this test into separate tests (which we normally do for tests with forms), testing each of the three scenarios above, that'd be fantastic.